When properly designed and constructed, rechargeable alkaline or alkaline-earth batteries, in particular lithium cells, can exhibit excellent charge-discharge cycle life, little or no memory effect, and high specific and volumetric energy.
In conventional lithium secondary batteries, particles of inorganic metal oxide such as lithium cobaltate (LiCoO2) or lithium manganese oxide (LiMnO2), generally mixed with conductive carbon black filler, are bound by a redox-inactive binder such as polyvinylidene fluoride and molded for being used as positive (cathode) electrode.
In recent years, secondary batteries having increased high energy density have come to be required, and organic materials have attracted attention as positive electrode materials capable of achieving such results.
Since conjugated electrically conductive polymers have been used as electrode materials for secondary batteries, much effort has been directed towards the development of this type of batteries. Actually, polymer batteries, i.e. batteries wherein an organic polymer is used as electrode, are expected to have many advantages, such as lighter weight, higher voltage, multiple shape capabilities, and a pollution-free construction, owing to the nature of the polymers.
Nevertheless, these polymer electrodes still have several drawbacks. In particular, electroactive polymers such as polypyrroles and polythiophenes generally possess unsatisfactory durability, poor cyclability and low oxidation potential values.
A system endowed with best current performances is based on the use of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) moieties as side chain of polymeric backbones. Main drawback of these systems are their limited oxidation potential, their slow kinetic of electron transfer and their low capacities, partly related to the need of incorporating in the polymer electrode substantial amounts of carbon black (generally about 50% of the overall electrode material) for ensuring suitable electronic conductivities.
Within these approaches, polymers comprising phenothiazine moieties have been considered as electrode materials.
In particular, WO 83/02368 (CHERON RESEARCH COMPANY) Jul. 7, 1983 discloses secondary batteries incorporating at least one electroactive organic polymer electrode, wherein said electroactive organic polymer is capable of undergoing a reversible oxidation or a reversible reduction to a charged conductive state, in which it exhibits a considerable stability. Among polymers which are reversibly oxidizable (p-type polymers) and which are thus especially well suited for use as cathodes, mention is made of certain fused 6,6,6-membered ring system polymers, among which those comprising diradicals of N-alkylphenothiazine are listed.
Properties of charge/discharge characteristics of various conductive polymers in lithium secondary battery assemblies are disclosed in NISHIO, Koji, et al. Characteristics of a lithium secondary battery using chemically-synthesized electrical conductive polymers. Journal of Power Sources. 1991, vol. 34, p. 153-160. In this investigation, a polyphenothiazine polymer of formula

was used as cathode in a Li/polymer cell; nevertheless, cell voltage was found to reach 5.0 V immediately after charging started, failing thus to provide stable charge/discharge curves.
It is also known the use of phenothiazine compounds as redox shuttles in conventional Li batteries for protecting electrodes against surcharge.
Thus, WO 2006/124738 (3M INNOVATIVE PROPERTIES) Nov. 23, 2006 discloses lithium-ion cells comprising, inter alia, electrolyte having dissolved therein N-substituted or C-substituted phenothiazine compounds serving as cyclable redox shuttle for protecting cell against overcharge.
Nevertheless, there is currently a shortfall in the art for rechargeable lithium-ion cells having improved cyclability, high capacity and high voltage outputs in combination with lightweight and environmental friendliness.